A typical RF excited CO2 slab waveguide laser is described in U.S. Pat. No. 5,123,028, assigned to Coherent, Inc., USA. A pair of rectangular planar electrodes having exposed light reflecting surfaces are spaced apart and dimensioned in a manner to guide light i.e. a waveguide, in a plane perpendicular to the reflecting surfaces. Light parallel to the reflecting surfaces is not constrained other than by resonator mirrors located at the ends of the electrodes. The resonator structure is designed as a negative branch unstable resonator in the non-waveguide dimensions. A stable resonator is used in the waveguide dimension but the mirror spacing from the end of the guide is based in part on the configuration of the unstable resonator. More particularly, the electrode length is selected and the mirrors are positioned such that the radius of curvature of the wavefront of the laser beam in the stable waveguide resonant cavity at the mirror location substantially matches the radius of curvature of the mirrors selected for the unstable resonator. Indeed this is true for all lasers; otherwise the path would not self repeat and you would not have a laser. We understand this patent to mean that the wavefront radius of curvature at the mirror surface substantially matches the mirror radius of curvature for the case of a substantially planar wavefront exiting and therefore re-entering the waveguide.
The object of the aforementioned design is to provide a CO2 slab waveguide laser which is stable and generates a high power output for a given length. Further features of the design are noted in respect of having a resonator spaced away from the ends of the waveguide to reduce degradation, having an improved electrode support structure which allows for thermal expansion of the electrodes, an improved cooling system, mirror mounts which allow adjustment from outside the laser housing, a means for pre ionising the discharge and having an electrode support structure which does not confine the discharge. While this design achieved high energies at fast repetition rates, the mode quality provided an M2 of 1.2 in both the waveguide axis and the unstable resonator axis. No further measures were taken to improve the mode quality from the device.
U.S. Pat. No. 6,856,639 assigned to Gosudarstvennoye Predpriyatie Nauchnoissledovatelsky Institut Lazernoy Fizike, Russia and Amada Company, Limited, Japan discloses a high power slab type gas laser. In this patent, they believe that the earlier laser described in U.S. Pat. No. 5,123,028 limited the output power due to the requirement that the electrodes could not be spaced more than 2 mm apart. They considered that the 2 mm restriction was required to generate the fundamental mode in the volume of the narrow, waveguide, axis and consequently, this restriction to the volume of the laser active region reduced the output power that could be obtained.
In U.S. Pat. No. 6,856,639, there is disclosed a gas laser comprising a pair of elongated electrodes arranged to define the discharge region between two opposing surfaces of said elongated electrodes, wherein the discharge region defines a longitudinal axis, a wide axis and a narrow axis. The gas laser further includes a laser gas disposed in said discharge region and an excitation means for energising the electrodes to excite the lasing gas. A first mirror is arranged in front of a first end of the pair elongated electrodes, wherein the first mirror is spaced apart from the first end along the longitude axis by a first distance, and a second mirror is arranged in front of a second end of the pair of elongated electrodes. Moreover, the two opposing electrode surfaces define an electrode curvature, respectively, that is adapted such that a wavefront of the fundamental transverse radiation mode in respect to the narrow axis substantially coincides with the mirror curvature at the first mirror at the first distance.
This arrangement allowed the fundamental transverse mode to operate through the electrodes while allowing the electrodes to have a separation with a minimum gap of approximately 2.5 mm to 3.7 mm. This gap increases to 3.5 mm to 6.00 mm at the ends of the electrodes. While this arrangement achieved good mode selection, this was due to there being no waveguide in the design with free space propagation of the fundamental mode provided in both axes, with an aperture formed between the electrodes to preferentially select the fundamental mode. They did not show that power increased with volume of active laser region. A major disadvantage to this design is that the electrodes are difficult to manufacture as a curvature must be machined to a high tolerance on each of the electrodes. This tolerance must also be applied in the arrangement of the mirrors around the structure and thus any miscalculation or misalignment can substantially affect the quality of the beam output. Additionally, thermal effects will vary the curvature of the electrodes thus affecting the predicted mode quality.
U.S. Pat. No. 5,216,689, being a continuation of the above mentioned U.S. Pat. No. 5,123,028, also considered profiling the electrodes. In one embodiment, extensions are formed at the ends of the electrodes between which the discharge is minimised. The extensions form recombinant surfaces between the ends of the electrodes and the mirrors to quench oxidizing species generated by the discharge before they reach the mirrors. This arrangement is considered to prevent degradation of the mirrors and had no effect on the mode selection performance of the laser.
U.S. Pat. No. 5,892,782, assigned to Synrad Inc., USA describes a laser which includes a split-wave hybrid resonator that produces a high quality laser beam from a low gain laser medium. The split-wave hybrid resonator includes a resonator cavity formed by a pair of resonator mirror surfaces positioned at opposite ends of the laser medium and a pair of resonator walls positioned on opposite sides of the resonator cavity. The resonator walls are separated from each other by a separation distance such that the resonator cavity has a Fresnel number between approximately 0.5 and 1.5.
At least one of the resonator walls includes a first ring oscillation filter adjacent to the lasing medium to filter out ring oscillations within the laser medium. This filter may take the form of a recess formed in one or both resonator walls. One or more of the resonator walls may include first and second wall portions angled with respect to each other to form a wave-front splitting interferometer. The resonator mirrors are tilted off-axis with respect to the resonator walls. This laser does not have electrodes arranged to form a waveguide. Additionally, the filter acts to stop a parasite rather than give mode selection.
U.S. Pat. No. 4,710,941, assigned to The United States of America as represented by the Secretary of the Army, describes a CW CO2 waveguide laser in which the electrodes have been profiled or more particularly machined to provide a plurality of apertures therein equally spaced along the electrodes. These perforated electrode structures are used to permit a large number of excited molecules normally trapped between the exciting electrodes to escape, allowing ground state molecules to enter and be pumped to their upper lasing level, thereby increasing the population inversion and efficiency of the laser. The apertures in this arrangement are of pinhole dimensions and are arranged in order that they do not influence the discharge or beam quality. The apertures do not provide any mode selection to the laser.
It is an object of at least one embodiment of the present invention to provide a laser in which the fundamental mode is preferentially selected.
It is a further object of at least one embodiment of the present invention to provide a laser of a known configuration in which a recess is provided in at least one of the surfaces of a waveguide.
It is a yet further object of at least one embodiment of the present invention to provide a method of constructing a mode selective laser.